The report of the 20th National Congress of the Communist Party of China emphasizes that promoting high-quality economic development is the primary task in building a modern socialist country in all respects, while digital economy agglomeration serves as an important driving force and an essential pathway to achieving this goal. However, “agglomeration can be both a blessing and a curse,” and how digital economy agglomeration influences high-quality development through spatial effects and underlying mechanisms remains an unexplored topic. This study first measures the levels of digital economy agglomeration and high-quality development across 282 Chinese cities from 2011 to 2019. It then employs the Spatial Durbin Model (SDM) to empirically examine the spatial effects and transmission mechanisms of digital economy agglomeration on high-quality development. The results reveal that digital economy agglomeration exerts significantly positive “local effects” and “neighboring effects” on high-quality development, demonstrating a pattern of “spatial resonance,” although such geographically proximate effects exhibit a wave-like attenuation with increasing distance. Mechanism analysis further shows that digital economy agglomeration promotes both local and neighboring high-quality development through green technology spillovers and market integration, while it facilitates the inflow but suppresses the outflow of innovation factors across regions. Based on these findings, this study provides policy implications for advancing digital economy agglomeration, fostering coordinated regional high-quality development, and promoting Chinese-style modernization.